A rotary valve of this kind is known from DE-A-41 35 593, the disclosure of which is hereby incorporated by reference in its entirety. The housing according to this document has a slot-like venting opening before the supply opening when seen in direction of rotation of the cellular wheel. Through this venting opening, leakage air from the upward moving cells void of bulk material is discharged, thus relieving the cells to the pressure prevailing in the supply opening or chute in order not to impede renewed supply of bulk material.
In order to prevent a relief of the cells in a sudden burst and to diminish noise emissions from the operating rotary valve, the lower edge of the perimeter of the venting opening situated on the peripheral wall of the cavity of the housing is slightly arrow-shaped to the longitudinal axis of the cellular wheel so that the relief of the cells, when the cellular wheel is rotating, is effected through a free cross-section of the venting opening which enlarges upon rotation of the wheel and communicates increasingly with the cell to be relieved.
Such a construction leads to satisfying results only with rotary valves which convey the bulk material against a relatively low pressure difference (3.5 bar for example), because the free cross-section of the venting opening enlarges quite quickly, when the cellular wheel rotates, thus causing a relief burst if the counter-pressure is high.